1. Field of the Invention
The present invention relates to antennas, and in particular, to a small surface-mountable broadband antenna.
2. Description of the Related Art
A helical antenna is disclosed in Japanese Unexamined Patent Application Publication No. 2003-37426 (Patent Document 1) as a small antenna that is used in mobile communication, such as cellular phones. The helical antenna enables operation in two frequency bands by winding an excitation coil around a long and narrow insulating main body in a helical fashion and winding first and second non-feeding coils around the main body in a helical fashion so that the first and second non-feeding coils are located adjacent to the excitation coil.
However, the spacing between the two frequency bands, in which the helical antenna can operate, is equal to or greater than several hundreds of megahertz, and the two frequency bands cannot be set close to each other so that the spacing is equal to or less than about 100 MHz. Moreover, although the band width of each frequency band is broad as compared to that of a helical antenna including a single coil, a sufficiently broad band width cannot be achieved.